Lab 2: Minerals I - Silicate Minerals

 

Isolated Tetrahedra

 

Olivine

 

Chemical formula: (Mg,Fe) 2 SiO 4

 

Structure: Iron and magnesium atoms are cations (positively charged). The tetrahedron is an anion, with its overall -4 charge. The tetrahedra are bonded to the iron and magnesium atoms because of the way the octet rule can be satisfied. The tetrahedra are totally isolated from one another, as the iron and magnesium atoms take up the space between the tetrahedra, where they form bonds with the tetrahedra.

 

Hammer behavior: When struck with a hammer, olivine fractures conchoidally (with curved fractures, as seen in a broken coke bottle). This reflects the evenness of the strength of bonds in olivine, where there aren't planes of weakness along which the hammer blow could produce shear faces (cleavage faces).

 

Color: usually olive-green, but it the iron can oxidize ("rust"), giving a reddish tinge

 

Streak: white

 

Luster: vitreous (glassy)

 

Diaphaneity: transparent to translucent

 

Hardness: 6.5 to 7, on a scale of 1 to 10, with diamond being 10; this is called Mohs Scale of Hardness:

 

Occurrence: forms much of volume of Earth's mantle; found in mafic (iron and magnesium rich) igneous rocks; also in moon rocks, Mars rocks, meteorites.

 

Garnet

 

Chemical formula: similar to olivine's

 

Structure: similar to olivine's

 

Hammer behavior: conchoidal to uneven fracture; often brittle; no cleavage (like olivine, no planes of weakness)

 

Color: most commonly red in color but can be found in a variety of colors, including purple, red, orange, yellow, green, brown, black, or colorless

 

Streak: white

 

Luster: vitreous to resinous (resin like)

 

Diaphaneity: transparent to translucent

 

Hardness: 6.5 to 7.5

 

Occurrence: metamorphic rocks

 

Single Chains of Tetrahedra

 

Augite (Pyroxene Group)

 

Chemical formula: (Mg,Fe) 2 SiO 3

 

Structure: single chains of tetrahedra bonded together with magnesium and iron atoms.

 

Hammer behavior: The bonds between the magnesium or iron atoms and the chains of tetrahedra are weaker than the strong covalent bonds holding the chains together. For this reason, a hammer blow will cleave the structure between the chains, and not across the chains. The angle of cleavage faces is at right angles, giving augite a blocky appearance in hand specimen:

 

Structure and cleavage of augite

 

Color: dark green to black, sometimes brownish

 

Streak: greenish-gray

 

Luster: fairly dull, although some vitreous specimens give the name augite, which in Greek means "brightness"

 

Diaphaneity: translucent to opaque

 

Hardness: 5.5 to 6

 

Occurrence: mainly found in mafic (magnesium and iron rich) igneous rocks such as gabbro and basalt

 

Double Chains of Tetrahedra

 

Hornblende (Amphibole Group)

 

Chemical formula: Ca 2 (Fe,Mg) 5 Si 8 O 22 (OH) 2

 

Structure: double chains of tetrahedra bonded together with magnesium and iron atoms

 

Hammer behavior: The bonds between the iron and magnesium atoms are weaker than those within the double chains. Just as augite has good cleavage, so does hornblende, but in hornblende specimens the cleavage angles are a combination of wide (about 120 degree) and narrower (about 60 degree) angles. Instead of having a blocky appearance, as with augite, hornblende specimens have a "splintery" appearance. Observe the cleavage angles in the illustration:

 

Structure and cleavage of hornblende

 

Color: generally dark, as greenish or brownish black

 

Streak: white

 

Luster: vitreous to pearly

 

Diaphaneity: translucent to opaque

 

Hardness: 5 to 6

 

Occurrence: mainly found in mafic igneous rocks, but is found more generally as an accessory mineral in igneous rocks; also common in certain types of metamorphic rocks.

 

Sheets of Tetrahedra

 

Biotite

 

Chemical formula: K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2

 

Structure: sheets of tetrahedra, with sheets sandwiched together with bonds to potassium, magnesium, iron, and aluminum atoms.

 

Hammer behavior: You don't even need a hammer; usually you can peel thin cleavage sheets with your fingernails (these aren't individual sheets, as atoms are tiny!).

 

Color: dark brown, greenish brown

 

Streak: gray

 

Luster: vitreous to pearly

 

Diaphaneity: transparent to opaque

 

Hardness: 2.5 to 3

 

Occurrence: found in igneous rocks such as granite

 

Muscovite

 

Chemical formula: KAl 2 (AlSi 3 O 10 )(OH) 2

 

Structure: sheets of tetrahedra, with sheets sandwiched together with bonds to potassium and aluminum atoms.

 

Hammer behavior: same as biotite

 

Color: white to gray, with various tones such as brown, green, silver

 

Streak: white

 

Luster: vitreous

 

Diaphaneity: transparent to translucent

 

Hardness: 2 to 2.5

 

Occurrence: found in igneous rocks such as granite

 

Framework of Tetrahedra

 

Orthoclase (Potassium feldspar)

 

Chemical formula: KAlSi 3 O 8

 

Structure: three dimensional framework of tetrahedra, with potassium and aluminum atoms bonded between the tetrahedra

 

Hammer behavior: Bonds are strong within the framework, but in some planes there are weaker bonds, leading to cleavage at 90 degrees. Orthoclase tends to break into blocky specimens. There are two directions of cleavage, not three. Look at a cardboard box and observe that if you put your hand on top of the box, that is the same plane as the bottom of the box. Take a pen or marker and place a check on the top and bottom of the box. Now move your hand from the top of the box to one of the sides; there is an opposite side of the box that is also in the same plane. Place a check on these two sides. Now, as you move the box around, you will see that there are two unmarked sides. If you look at specimens of orthoclase, you may find cleavage angles at 90 degrees between any two cleavage planes, but like the cardboard box, when you look at the "ends" relative to those two planes, you will not see cleavage. The specimen will not show super smooth surfaces here, and will show uneven fracture.

 

Color: often pinkish, to white, to colorless

 

Streak: white

 

Luster: vitreous

 

Diaphaneity: transparent to translucent

 

Hardness: 6

 

Occurrence: igneous rocks, and metamorphic rocks

 

Plagioclase (Calcium/sodium feldspar)

 

Chemical formula: (Ca, Na)AlSi 3 O 8

 

Structure: three dimensional framework of tetrahedra, with calcium, sodium, and aluminum atoms bonded between the tetrahedra

 

Hammer behavior: Same as orthoclase, but plagioclase can have well-developed striations

 

Color: white to gray, with bluish, greenish, or reddish tinting

 

Streak: white

 

Luster: vitreous

 

Diaphaneity: transparent to translucent

 

Hardness: 6 to 6.5

 

Occurrence: igneous rocks

 

Quartz

 

Chemical formula: SiO 2

 

Structure: three dimensional framework of tetrahedra, in pure forms usually without atoms of other elements, but there are commonly other elements

 

Hammer behavior: Bonds in the framework of quartz are strong in all directions, so when hit with a hammer, even if the specimen is a beautiful crystal, it just fractures conchoidally.

 

Color: highly variable

 

Streak: white

 

Luster: vitreous

 

Diaphaneity: transparent

 

Hardness: 7

 

Occurrence: all rocks

 

Other Silicate Minerals

 

Talc

 

Chemical formula: Mg 3 Si 4 O 10 (OH) 2

 

Structure: foliated (alignment of crystals from pressure) to masses of tiny crystals

 

Hammer behavior: Talc is very soft, so it comes apart into pieces easily. Technically, there is a perfect direction of cleavage when you can see it.

 

Color: white with various tintings, to greenish

 

Streak: white

 

Luster: vitreous to pearly

 

Diaphaneity: translucent

 

Hardness: 1

 

Occurrence: in metamorphic rocks

 

Kaolinite

 

Chemical formula: Al 2 Si 2 O 5 (OH) 4

 

Structure: clay-like masses, appearing smooth

 

Hammer behavior: Kaolinite is very soft, so it comes apart into blocky, irregular pieces easily.

 

Color: white, with various tintings

 

Streak: white

 

Luster: earthy (dull)

 

Diaphaneity: transparent to translucent

 

Hardness: 1.5 to 2

 

Occurrence: weathered aluminum-silicate rocks

 

Antigorite (Serpentine)

 

Chemical formula: (Mg,Fe) 3 Si 2 O 5 (OH) 4

 

Structure: occurs in masses of small crystals, in blocky specimens displaying brittle fracture

 

Hammer behavior: See comment above; in microscopic view, can see good cleavage in one direction.

 

Color: green, gray, black

 

Streak: greenish white

 

Luster: vitreous to greasy

 

Diaphaneity: transparent to opaque

 

Hardness: 3.5 to 4

 

Occurrence: metamorphic rocks